1. Field of the Invention
The present invention relates to a video disk on which a video signal and a digital data signal are multiplex recorded.
2. Background Art
Recently, disks on which information is densely recorded have been developed and commercialized. Typical examples of such disks are video disks and digital audio disks. There are various systems for recording such disks. In the case of optical video disks, a carrier of 8.1 MHz for a television video signal is frequency-modulated so as to make a synchronization level and a white level of the video signal be 7.6 MHz and 9.3 MHz for recording of the television video signal. Also, a carrier of 2.3 MHz and 2.8 MHz for an audio signal accompanying the television video signal, which is used as a stereophonic signal, a bilingual signal, or the like, is frequency-modulated to thereby record the audio signal. In the case of optical digital audio disks, on the other hand, a spectrum of an EFM (Eight-to-Fourteen Modulation) signal of a pulse-code-modulated (PCM) stereophonic audio signal occupies a frequency band lower than about 2 MHz. Since such a frequency band below about 2 MHz is empty in a video disk, it is possible to record such an EFM signal as described above onto the video disk by frequency-divisionally multiplexing the EFM signal.
On such video and digital audio disks as described above, time codes such as AMIN, ASEC, AFRAME, (minute, second and frame) or the like, used as absolute addresses, are recorded for every television frame or for every subcode frame. In a video disk, for example, the frequency of the television frame is selected to be 29.97 Hz (in the case of the NTSC system), and therefore the time code is incremented by one second every 30 frames from the initiation of a program. The frame numbers from 0 to 29 are respectively added to the 30 television frames belonging to the same second. That is, for example, the 29th frame of the 10th second is followed by the sequence of the 0th frame of the 11th second, the 1st frame of the 11th second, the 2nd frame of the 11th second, and so on.
In a digital audio disk, on the contrary, the time code is recorded for every subcode frame (block) at a rate of 75 Hz. In this case, 75 subcode frames (75 blocks) correctly correspond to 1 second, and therefore the time code is incremented by one second for every 75 subcode frames. The frame numbers from 0 to 74 are respectively added to the 75 subcode frames belonging to the same second. That is, for example, the 74th frame of the 10th second is followed by the sequence of the 0th frame of the 11th second, the 1st frame of the 11th second, and so on.
The frequency of a subcode frame of such a digital data signal as described above is selected to be 75 Hz so that a time code of each subcode frame is coincident with actual time. That is, 1 second accurately elapses while the data signal of 75 subcode frames is reproduced. The frequency of a television frame of the video signal, on the other hand, is selected to be 29.97 Hz, and therefore a time code of each television frame is not exactly coincident with actual time. That is, an error of about 3.6 second is generated for one hour of actual playing time. Since the above-mentioned error is small, there has been no disadvantage in practical use when only the video signal is recorded on a video disk and retrieval is performed only by the time code of the video signal. However, when the digital data signal is successively and continuously recorded on the video disk by being superimposed on the video signal and the respective time codes of the digital data and video signals are recorded on the video disk, the retrieved position when the time code of the digital data signal is retrieved is different from that when the time code of the video signal is retrieved. As a result, the practical use of the video disk is degraded. The displacement in subcode frame reference time corresponds to about 270 subcode frames in 60 minutes of video frame reference time which is counted on the assumption that 30 television frames correspond to 1 second.
Recently, the inventor of this application has proposed a process in which the second timing of one of the respective time codes of a television frame and a subcode frame is incremented by one at a television frame or a subcode frame newly initiated only after the second timing of the other time code has been incremented by one (Japanese Patent Application No. 146985/85 and U.S. patent application, Ser. No. 882,002, filed July 3, 1986). In this proposal, however, there has been an inconvenience that it is necessary to use a signal synchronized with the television frame or the subcode frame in order to edit a digital data or a video signal to be recorded onto a video disk.